Mass number of the most abundant of the stable nuclides with atomic number equal to n. If there is no stable isotope, choose the one with the longest decay time.

Sequence is not well-defined, since the entries may change as more properties of the elements are discovered.

Moss and Winter give the atomic weight for Dysprosium (Dy) as "162.50(3)"; rounded to 163 rather than 162.

Bentor, Winter and Moss all give 145 as the atomic weight of the most stable isotope of Promethium (Pm), as opposed to 147, which was in the original data (between 144 and 150).

Also referred to as the mass number A of the most abundant and stable nuclide with atomic number Z=n. We have A = Z + N, where N is the neutron number A058317(n). - Lekraj Beedassy, Oct 08 2005

The 'abundance' refers to the Earth's crust. It may differ when applied to other astronomical bodies and/or to the known Universe. The mass numbers (once also referred to as the isotope numbers) are always integer, and should not be confused with relative atomic masses (or isotope masses), nor with (mean) atomic weights, none of which are ever integer. - Stanislav Sykora, Apr 26 2015

Andreas von Antropoff was an Estonian-born German chemist, who is known to have coined the term Neutronium and developed a temporarily and widely used periodic table of elements in 1926. - Jinyuan Wang, Apr 30 2019

Calcium (Ca), with atomic number 20, has 6 stable isotopes with mass numbers 40, 42, 43, 44, 46, and 48. Of these, 40Ca is the most abundant in Earth's crust (96.941%). Hence a(20)=40. Note: The relative atomic mass of 40Ca is 39.96259..., smaller than 40 because of the mass equivalent of the nuclear bonding energy, while the conventional mean atomic weight of Calcium is presently 40.078(4), by IUPAC 2009. - Stanislav Sykora, Apr 26 2015

Comments from David Terr: There are several errors in this sequence. It looks to me that the rounded atomic weights were used rather than the mass number of the most abundant of the stable nuclides with atomic number equal to n. Thus for instance, a(28) should be 58, not 59 and a(29) should be 63, not 64. In fact, in all cases the parity of a(n) should be the same as n (both odd or both even). - David Terr, Oct 05 2006

I fixed all the entries I could find in which a(n) was incorrect, based on the online table of isotopic abundances at http://www.sisweb.com/referenc/source/exactmas.htm. - David Terr, Apr 06 2009